CSC231 Homework 4

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Assignment #4

The assignment is due on 10/8/09 at 11:59 p.m. plus 1 minute.

Problem #1: Addressing modes and one-dimensional arrays

Assume that your data section is initialized with the following variables:

     msg     db    3,"ALL",11,"PROGRAMMERS",3,"ARE",11,"PLAYWRIGHTS"
             db    3,"AND",3,"ALL",9,"COMPUTERS",3,"ARE",5,"LOUSY"
             db    6,"ACTORS"
     N       equ   10

Note that the msg contains byte numbers followed by words. The byte number preceeding a word corresponds to the number of letters in the word. So '3,"ALL"' for example means that the word that follows 3 is a 3-character word. '11,"PROGRAMMERS"' means that the string following the byte that contains 11 is an 11-character word.

N represents the total number of words in the string msg.

Your assignment is to write a program that uses one or several loops that go N times and that print all the words on separate lines, one after the other, such that each word is capitalized, i.e. its first letter is uppercase, and the others are lowercase.

Save your program in a file called hw4a.asm which you will submit as follows:

          submit  hw4  hw4a.asm

Problem #2: Pascal's Triangle

Preparation

While printing strings of characters on the screen is relatively easy in assembly, as you saw in the previous assignment and exercises, inputing numbers and printing numbers is more difficult in assembly (we'll see why in class).

So, for the time being, to deal with the input/output of integers we will use a trick: we will link our assembly language program to a C program, and make the C program handle the input and output of information for us.

Don't worry if you don't know C. We will not write a single line of C code except for the small C program which will never change.

When we want to print a number from our assembly program, or when we want to get a number from the keyboard, we simply call the appropriate C function from our assembly program. Because the linker will link our program with the C program, we can call functions written in another language from assembly. This is a useful programmer trick!

It sounds complicated, but you'll see that it isn't that bad.

Let's try this concept with a simple example:

First use your favorite editor to write the following C program and call it driver.c

     #include <stdio.h>

     extern int asm_main( void );

     int main() {
       asm_main();
     }

All it does is have a main program that calls a function asm_main(), which will be the entry point of our assembly language program.

Next get a small library that will provide a simple interface between this C program and our assembly language program.

• asm_io.asm
• asm_io.inc

Create the object file for the library:

     nasm -f elf -F stabs asm_io.asm

Now enter the test program below in assembly. Call it hw4test.asm. Be careful that it is slightly different from the ones we have been writing so far in that the starting label and the last instructions are not the standard ones.

     ;;; hw4test.asm
     ;;; a simple demo program to test I/O with
     ;;; a C "wrapper" program

     %include "asm_io.inc"

             ;; -------------------------
             ;; data segment
             ;; -------------------------
             section .data
     msg     db      "Hello! Please enter an integer number: ",0x00
     msg2    db      "You have entered ",0x00
     x       dd      0

             ;; -------------------------
             ;; code area
             ;; -------------------------
             section .text
             global  asm_main
     asm_main:       

             mov     eax,msg               ; pass address of the string msg in eax
             call    print_string          ; call a function in asm_io library
                                           ; that will print the string msg

             call    read_int              ; call a function that reads an
             mov     [x], eax              ; integer from the keyboard and
                                           ; returns it in eax.  Save returned
                                           ; value in variable x

             mov     eax,msg2              ; print the string msg2
             call    print_string          ; ("you have entered")

             mov     eax,[x]               ; get integer user entered
             call    print_int             ; pass it via eax and call the
                                           ; function specialized in printing
                                           ; integer numbers

             call    print_nl              ; call the function that prints
                                           ; a new blank line

             ;; return to C program

             ret

Note: the beginning of the program is not labeled _start any longer, but asm_main. The reason is that the start of the program is in the driver.c program, not in our assembly program. Also, the program does not end with an int 0x80 instruction any longer, but by a ret instruction, so that it can return to the C program which will end with its own (hidden) int 0x80.

Although we haven't seen the call instruction yet, it's fairly easy to understand what it does. It forces the processor to go somewhere else in the memory, execute a piece of code, and return back to the program. We'll look at it in details later. Right now the logic of the program should be sufficient for you to see how to use these simple functions to print strings and number.

Note: in C all strings of characters must be terminated by a 0 (0x00) byte. Because here we have used a C function to print the strings of characters, we added a 0x00 byte at the end of each string. Make sure that you add 0x00 at the end of all your strings of characters if you are going to use the method shown above to print your strings.

To assemble and run your program just follow these steps:

            nasm -f elf -F stabs hw4test.asm
            gcc -o hw4test driver.c asm_io.o hw4test.o 
            ./hw4test

            Hello! Please enter an integer number: 1234
            You have entered 1234

The first line assembles hw4test.asm. The second line asks the C compiler gcc to compile driver.c and to link it with the object files asm_io.o and hw2test.o, and to store the executable in a file called hw4test. The last line executes the program.

The functions provided by the asm_io.o library are described now:

• read_int: reads an integer from the keyboard and stores it into the EAX register.

• print_int: prints out in 2's complement the value of the integer stored in EAX on the screen.

• print_string: prints out to the screen the contents of the string at the address stored in EAX. The string must be terminated by a 0x00 byte (C-string).

• print_char: prints out to the screen the character whose ASCII value is stored in AL

• read_char: reads a single character from the keyboard and stores its ASCII code into the EAX register.

• print_nl: prints out to the screen a new line character.

The asm_io library supports also two macros that you may find useful: dump_regs and dump_mem. They can be used to display the contents of the registers and the contents of memory. Try adding these two lines at the end of your hw4test.asm program:

             call    print_nl              ; call the function that prints
                                           ; a new blank line

             dump_regs 1
             dump_mem  1, msg, 3

              
             ;; return to C program
             ret

The first of the new lines dump_regs 1 dumps the registers and uses 1 as a label, so that if you use this macro several times in your program, you can label each one with a different number so that you can figure out from the output which macro actually output what. The second of the new lines dump_mem 1, msg, 3 output 3 lines of 16 bytes starting at the address corresponding to the variable msg, and uses 1 as the label for this output.

Programming Assignment

Write a program that outputs the first 10 rows of Pascal's Triangle.

Your program should use an array and initialize it with 1 in the first cell, and 0 everywhere, else. Then your program prints it. It's the first row of Pascal's triangle:

1 0 0 0 0 0 0 0 0 0

Then scan the array starting with the last cell (using a register pointer) and replace this cell with the sum of itself and its left neighbor:

   1 0 0 0 0 0 0 0 0 0
                    \|                    0+0 = 0
   1 0 0 0 0 0 0 0 0 0

Keep on going "down" the array by moving the pointer left by one cell (remember that if your array is an array of word, that means decrementing the register by 2, if an array of double words, decrementing it by 4), summing up cells with their left neighbors:

   1 0 0 0 0 0 0 0 0 0
                  \|                  0+0 = 0
   1 0 0 0 0 0 0 0 0 0


   1 0 0 0 0 0 0 0 0 0
                \|                    0+0 = 0
   1 0 0 0 0 0 0 0 0 0


   1 0 0 0 0 0 0 0 0 0
              \|                      0+0 = 0
   1 0 0 0 0 0 0 0 0 0

   . . .

   1 0 0 0 0 0 0 0 0 0
    \|                                1+0 = 1
   1 1 0 0 0 0 0 0 0 0

You end up with the second row of Pascal's triangle:

  1 1 0 0 0 0 0 0 0 0 0

which you can now print.

Repeat this process until you have printed all 10 rows of the triangle. Row 1 is the one with one 1 and nine 0s on it.

Your program should output the rows one above the other, including the zeros:

1 0 0 0 
1 1 0 0
1 2 1 0
1 3 3 1
...

Format your program so that it uses the driver.c and asm_io.asm programs to print integers. Store your program in a file called hw4b.asm, and submit it as follows:

   submit hw4 hw4b.asm

Hints

1. Figure out what the largest number you will get is, and decide whether you want to use an array of bytes, words, or double words.
2. You may want to start by initializing your array to Pascal's Row 4, for example, and transform it into Row 5, using just one loop that scans the array. Once this works, you can then add an outside loop that will make the program compute the other rows, starting with Row 0.